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Physical Review A

Autoionizing states in strong laser fields

P. Lambropoulos, P. Zoller

Published Jul 1, 1981• 351 citations
Publisher

Abstract

A formalism is developed for the study of autoionizing states under the influence of strong lasers. It is cast in both a semiclassical as well as a fully quantum-mechanical form. A set of integrodifferential equations is derived and solutions are obtained for certain special cases. It is demonstrated that the strong field causes significant distortion of the line profile and that new effects appear in double optical resonance when autoionizing states are involved.

DOI
10.1103/physreva.24.379

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A formalism is developed for the study of autoionizing states under the influence of strong lasers. It is cast in both a semiclassical as well as a fully quantum-mechanical form. A set of integrodifferential equations is derived and solutions are obtained for certain special cases. It is demonstrated that the strong field causes significant distortion of the line profile and that new effects appear in double optical resonance when autoionizing states are involved.
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Previous studies on strong-field autoionization involving high-order ionization processes are mainly investigated for the cases of CW laser irradiations. This paper extends those studies to the cases of square laser pulses with arbitrary widths. Populations of the ground state and the line shapes of the autoionizing resonance are discussed. It is shown that strong-field autoionization and the high-order ionization processes are drastically affected not only by the intensities but also by the time of interaction between atoms and laser field.
A theory describing the radiative decay of an autoionizing state under strong pumping by a coherent field is developed. The theory systematically takes into account the radiative decay of the unperturbed continuum. The problem at hand corresponds to the case of strongly coupled bound states decaying to electron and photon continua with the two continua also weakly coupled to each other. A master equation describing the time evolution of the atomic system is derived, and its exact solution under arbitrary initial conditions is given. The effect of radiative decay on the Fano profiles and photoelectron spectra is analyzed in detail. The time development of the system is also examined. The radiative decay of the autoionizing state and the unperturbed continuum changes the spectra in a significant way. The characteristics of the spectra are correlated with the dressed states (with complex energies) of the system. The changes in the structure of the dressed states as a function of the system parameters such as the spontaneous-emission rate and laser intensity are discussed in detail.
We derive a parametrized formula for the photoionization cross section in the vicinity of an autoionizing state (AIS) coupled with another AIS by a strong laser field. The rotating-wave approximation is used and adiabaticity is assumed. In the probe field spectrum two Fano resonances contribute to the cross section, while a single resonance occurs in the strong-field spectrum. The correlation between the resonance shape and the complex energies of the laser-driven two-level system is described. Calculations are performed for the and AIS pair in He and for the and pair in Mg. The results agree with those obtained using time-dependent approaches. The formula for the cross section describes the spectral features of the ion yield measured in the two-photon ionization of the Mg atom via the AIS coupled with the state. The appearance of laser-induced degenerate states in the cross section shape is studied.
We present a model calculation for two-photon autoionization of H2{\mathrm{H}}_{2}H2​ to the $X^{1}\ensuremath{\Sigma}_{g}$ state of H2+{\mathrm{H}}_{2}^{+}H2+​. The theory is based on a formulation that allows for strong-field effects. It is shown that, as a result of such effects, the autoionization profile will in general exhibit qualitatively new behavior at laser intensities needed for two-photon ionization. The magnitude of these effects depends significantly on the presence of resonance with intermediate states. The notion of autoionization width and qqq parameter are generalized to include this new behavior. Detailed calculations are reported for intensity effects on the line profile, the role of intermediate resonances, and the separate contribution of the vibrational states of H2+{\mathrm{H}}_{2}^{+}H2+​ as final states. The feasibility of observation with currently available sources is discussed.
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